The present invention relates to a method and apparatus for low air pressure spraying. Particularly, but not exclusively, the invention is applicable to spray guns for the application of paint and like material surface treatments, particularly water-based paints.
The use of spray guns for application of paints is well known. However, it has been found that when water-based, high gloss paints are sprayed through a high pressure or conventional spray gun, the level of gloss is reduced. This is also true of the high volume-low pressure type of spray gun which operate at only 10 psi air cap pressure.
Tests carried out at various pressures have shown that the loss of gloss is due to air bubbles rising to the surface of the paint as it dries. It has been found that the greater the pressure used to spray the paint, the more air bubbles appear. The cause of the bubbles is that dissolved air is being released from the water as the paint dries. The greater the air pressure when the paint is sprayed, the greater the volume of dissolved air and the greater the number of bubbles.
If the air pressure is low but the volume is high, gloss levels are reduced. To achieve the desired gloss levels with this type of paint it is necessary to design a spray gun that will operate at very low air pressures and very low air volumes. It must achieve acceptable levels of atomization, have sufficient energy to transfer the paint at an acceptable rate to the surface of the target, and expand the natural cone of spray into a useful fan pattern.
In the past, spray guns have used air pressures between 40 and 90 psi, and these high pressures cause a cushion of air to be formed on the surface of the product being treated. This cushion causes some of the sprayed material to bounce back and be displaced laterally by the following airflow to be lost in the surrounding air.
Accordingly, this type of spray gun is very inefficient. Rarely are transfer efficiencies greater than 40% and more often nearer 30%. The waste paint material produces unacceptable emissions of volatile organic compounds and leaves a solid residue which can remain floating in the air for some time. These can be highly toxic and damaging to the atmosphere and health. To overcome these problems, it is necessary to reduce the air pressure and air volume used in such guns. Therefore, the environmental requirements for an acceptable spray gun are similar to those required for achieving a good gloss in water-based paints.
If the air pressure is reduced on a spray gun that was originally designed for high pressure use, the turbulence and restrictions in internal air passages and the air cap cause a loss of air speed and a reduction in air volume. The result of this is low paint transfer rates, poor atomization and an inferior paint finish. However, transfer efficiency is improved. If the air volume is increased while keeping the pressure low, the ratio of air to paint increases and the problems experienced with high pressure will return depending on the increase in volume.
Existing high pressure spray guns have been modified to operate at low pressures, but the complexity of the designs and the intricate interconnecting drilled passages do not permit good air flow. In an effort to overcome the poor performance, air cap ring gaps were increased, resulting in a substantial increase in air consumption. This type of spray gun has become known as the high volume-low pressure (HVLP) gun.
More specifically, in HVLP spray guns the means for actuating the control valves within the gun have had considerable shortcomings. For example, it is commonplace for the stem of the needle valve and its associated compression spring and housing to extend through the main air flow passage to the nozzle, thereby leading to significant restrictions in the air flow path.
Likewise, in order to provide a convenient means for actuating the stem of the air flow and fluid needle valves, the main nozzle of the apparatus is mounted on a forward projection of the apparatus so as to leave a free space to accommodate the arc of movement of the valve control trigger.
Moreover, since the same trigger operates both the liquid and air control valves, the progressive control from on to off operating characteristics of the air control valve can be restricted in certain operating conditions where the liquid control valve has been manually adjusted to such a point that it affects the ability of the trigger to operate both valves simultaneously through the full range of movement.
The object of the present invention is to provide a method and apparatus for spraying paint and other surface treatment liquids, offering improvements in relation to one or more of the matters discussed above, or generally.
According to a first aspect of the invention there is provided an apparatus for spraying liquid surface treatment material, said apparatus having a housing, a liquid inlet for supply of the liquid surface treatment material, a gas inlet for supply of pressurised gas to be mixed with the liquid surface treatment material, an outlet nozzle through which the gas and liquid surface treatment material is sprayed, a control valve adapted to regulate the supply of the liquid surface treatment material to the outlet nozzle, a gas valve operable between an open position and a closed position, a first communicating passageway connecting said gas inlet to said gas valve, and a second communicating passageway connecting said gas valve to said outlet nozzle; wherein said second passageway is provided with a stepped portion therein so that a gas vortex is created therethrough.
Preferably, said second passageway is offset from said first passageway. Preferably, said second passageway is substantially conical in shape. Preferably, said second passageway includes an inlet and an outlet, wherein said passageway is tapered from said inlet to said outlet. Preferably, said taper is between 1 and 15xc2x0.
Preferably, said stepped portion of said second passageway comprises a ledge whose width tapers up to a maximum of 10% of the radius of said second passageway at the level of the stepped portion.
Preferably, said second passageway has a radius of curvature at said outlet so as to provide gas to the nozzle in a substantially horizontal direction.
Preferably, the longitudinal axis of said outlet nozzle extends across said second passageway. Preferably, the axis of symmetry of said ledge is offset from said longitudinal axis of said outlet nozzle, thereby inducing a vortex in the air flowing through said passageway.
According to a second aspect of the invention there is provided an apparatus for spraying liquid surface treatment material, said apparatus having a housing, a liquid inlet for supply of the liquid surface treatment material, a gas inlet for supply of pressurised gas to be mixed with the liquid surface treatment material, an outlet nozzle through which the gas and liquid surface treatment material is sprayed, a control valve adapted to regulate the supply of the liquid surface treatment material to the outlet nozzle, a gas valve operable between an open position and a closed position, a first communicating passageway connecting said gas inlet to said gas valve, and a second communicating passageway connecting said gas valve to said outlet nozzle; wherein said second passageway is axially offset from said first passageway and is substantially conical in shape, and wherein said second passageway includes an inlet and an outlet and is tapered from said inlet to said outlet at an angle of taper of between 1 and 15xc2x0.
Preferably the apparatus further comprises a trigger means, whereby said trigger means is adapted to operate both of said control valve and said gas valve.
Preferably, said gas valve is an axially-sliding piston valve. Preferably, said control valve is a liquid control needle valve.
Preferably, said outlet nozzle is controlled by said liquid control needle valve.
Preferably, said piston valve produces an annular air jet in said second passageway. The piston valve may be tapered or parallel. In addition, an air control valve stem is provided which is connected to the piston valve and operated by said trigger means.
Preferably, said piston valve comprises inner and outer co-axial apertured sleeves, wherein said inner sleeve is located within said outer sleeve and is rotatably adjustable relative to said outer sleeve.
Preferably, the liquid control needle valve is controlled by said trigger means via an axially-sliding sleeve or slipper member situated on a rearward portion of the housing. Preferably, it is also provided with a rotational flow adjustment means to adjust the flow rate of the liquid.
Preferably, said flow adjustment means comprises a stem member, a rotational adjuster, and a return spring, said stem member being threaded at its rearmost extremity to accept the rotational adjuster. Preferably, said stem member is actuated externally by the trigger means, and is returned to its initial position by a return spring.
Preferably, the apparatus further comprises a regulating valve and a pair of side jets, whereby the spray pattern of the outlet nozzle is regulated by said regulating valve, and said side jets are utilised to regulate said spray pattern.
Preferably, the needle valve is supplied with the paint or material surface treatment liquid by a pressurized material supply connector which distributes the material via a radial port to said needle valve. Alternatively, the material may be introduced to the apparatus from a gravity liquid reservoir fitted to the uppermost aspect of the apparatus via a radial port.
According to a third aspect of the present invention, there is provided a method of spraying a fluid onto a surface, said method comprising the steps of:
supplying a liquid to be sprayed into a liquid inlet of a spray apparatus;
supplying a pressurised gaseous propellant into a gas inlet of said spray apparatus;
passing said gaseous propellant through a communicating passageway from said gas inlet to an outlet nozzle;
accelerating said gaseous propellant by creating a gas vortex as said propellant passes through said communicating passageway;
passing said accelerated propellant through an outwardly tapering portion of the communicating passageway to further accelerate the vortex and supply the propellant to the outlet nozzle in the form of an annular gas jet; and
spraying said liquid onto a surface by mixing said liquid and said annular gas jet at said nozzle.
Preferably, said passageway comprises an upper portion and a lower portion, wherein said upper portion is axially offset from said lower portion and is substantially conical in shape. Preferably, said upper portion of said passageway includes an inlet and an outlet and is tapered from said inlet to said outlet at an angle of taper of between 1 and 15xc2x0.
Preferably, the mixing of said liquid and said annular gas jet is controlled by a trigger valve mechanism on said spray apparatus. Preferably, said trigger valve mechanism comprises:
a gas valve operable between an open position and a closed position;
a control valve adapted to regulate the supply of the liquid to be sprayed; and
a trigger means;
whereby said trigger means is adapted to operate both of said gas and control valves.
Preferably, said control valve is a liquid control needle valve. Preferably, said gas valve is an axially-sliding piston valve. Preferably said piston valve comprises an inner apertured sleeve and an outer apertured sleeve, said inner and outer sleeves being co-axial, and wherein said inner sleeve is located within said outer sleeve and is rotatably adjustable relative to said outer sleeve.